Graduate Thesis Or Dissertation

Identifying factors driving sensitivity to fragmentation in forest breeding songbirds

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  • Habitat loss and fragmentation are the greatest threats to biodiversity worldwide. Fragmentation impacts landscape configuration, resulting in a larger number of patches that are smaller in size and further apart from one another. Island biogeography and metapopulation theory predict populations in these remnant patches should be smaller, have higher extinction rates, and be less likely to receive immigrants from other populations. However, empirical data frequently do not conform with these theoretical predictions, leading to assertions that this model is too simplistic to describe distributions and dynamics of fragmented populations. However, others believe that landscape configuration effects have been poorly tested and modeled to date. In this dissertation, I use breeding bird data collected in a fragmented forest landscape to explore this lack of congruence between theory and reality. I first test the hypothesis that heterogeneity in the detectability of mobile species due to temporary emigration from sample sites can produce biased estimates of metapopulation rates. Next, I test for idiosyncrasies in the effects of forest loss and fragmentation on species belonging to different ecological trait groups. Lastly, I examine whether fragmentation actually reduces the functional connectivity of landscapes for species identified as fragmentation-sensitive. Dynamic occupancy models are popular for estimating metapopulation dynamic rates (colonization and extinction) from repeated presence/absence surveys of unmarked animals. This approach assumes closure among repeated samples within primary periods, allowing estimation of dynamic rates between these periods. However, the impact of temporary emigration (reversible changes in sampling availability) on dynamic rate estimates has not been tested. In Chapter 2, I use simulated data to investigate the degree to which temporary emigration could mislead researchers interested in quantifying metapopulation rates. I then compared results from three avian point count datasets to evaluate the likelihood that temporary emigration confounds estimates of dynamics for 19 species under a popular sampling protocol. Simulated experiments indicated that when secondary periods were open to temporary emigration, presence of dynamics was identified ≥ 95.1% of the time, and dynamic rate estimates were accurate. However, dynamic rates were biased when secondary periods were closed to temporary emigration. In empirical datasets, dynamic occupancy models had greater support than closed models for all species when secondary sampling periods occurred in immediate succession (i.e., 3 samples within 10 minutes); however, my results suggest that this is because these estimates were heavily influenced by temporary emigration. When counts within a primary period were separated by 24-48 hours, I found evidence of dynamics for less than half of these species. I recommend an alternative sampling approach that allows accurate estimation of dynamic rates when temporary emigration is of no interest, and introduce a novel model for estimating both processes simultaneously in rare cases where they are both of biological interest. Concern for violating the occupancy modeling closure assumption has led to widespread recommendations that samples within primary periods be conducted extremely close in time. However, these results indicate this is not the best approach when interest is in quantifying dynamic rates. While dynamic occupancy models provide estimates of ‘colonization’ and ‘extinction,’ these values do not inherently represent dynamics unless temporary emigration has been explicitly modeled or accounted for with sampling design. Naivete to this fact can result in incorrect conclusions about biological processes. While theory predicts that fragmentation should negatively influence biodiversity, empirical support of this idea is weak in terrestrial systems. However, tests of fragmentation effects are typically confounded with landscape composition and potentially obscured by imperfect detection. In Chapter 3, I used multi-species occupancy models and a mensurative experimental design to test competing hypotheses about how forest fragmentation influences distributions of breeding forest bird species and communities. During the breeding seasons of 2011-2013, we recorded over 80,000 bird detections in 202 forest fragments using a sampling design that isolated the effects of patch size per se from the effects of forest amount (2 km), edge, local vegetation, and sample area. I modeled the effects of these covariates on distributions of individual species categorized by ecological trait groups (i.e., forest, forest interior, or forest edge). Though my results indicated little effect of patch size on total species richness, increasing patch size tended to have a positive effect on interior species, and a negative effect on edge species. The effects of total forest amount were much more variable, and actually had a negative influence on many species, particularly cavity nesters. My results do not support theoretical predictions that forest patch size should positively influence bird species richness. However, composition of bird communities does shift toward edge species from interior species with decreasing patch size. Maintaining large forest patches is thus critical for supporting forest interior species, which tend to be of greater conservation concern. Maintenance of metapopulations requires movement of dispersers among resource patches. The degree to which a landscape facilitates or impedes such movements is defined as functional connectivity. Habitat fragmentation may reduce the functional connectivity of a landscape, but empirical linkages between distribution patterns and movement ability are lacking. In Chapter 4, I use experimental translocations to test whether forest fragmentation impedes movement of two species identified as fragmentation-sensitive in Chapter 3: Wood Thrush (Hylocichla mustelina) and Ovenbirds (Seiurus aurocapilla). I also tested for behavioral changes in translocated birds and evaluated whether fragmentation effects differed between behavioral modes. Over two breeding seasons, we translocated 35 Wood Thrush and 19 Ovenbirds (1-1.2 km) across landscapes spanning a fragmentation gradient and recorded their movement paths using VHF transmitters and receivers. Eighty-seven percent of individuals returned successfully, taking as long as 72.2 hours. Movement patterns of 96% of successful birds indicated two distinct behavioral modes: exploring, characterized by short, undirected movements and course reversals; and homing, characterized by large, fast steps towards their home territories. Forest composition and configuration had no effect on homing time or path straightness for either species. However, at a finer scale, I found that both preferred to take steps that minimized their exposure to non-forested gaps. My results demonstrate that movement limitation could drive or exacerbate fragmentation sensitivity for these birds. Further, while fragmentation effects did not differ between behavioral modes, my results highlight the need to link the dichotomous behaviors of translocated animals with natural movement processes. Despite this knowledge gap, results from our study suggest that maintaining contiguous habitat or corridors may improve functional connectivity for fragmentation-sensitive birds.
  • Keywords: forest fragmentation, dispersal, habitat amount, dynamic occupancy models, edge effects, extinction, metapopulation, patch size, colonization, temporary emigration, island biogeography, translocation experiment, functional connectivity, gap crossing behavior, movement behavior, detection probability, closure assumption, community occupancy model, breeding bird, Pollock's robust design
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